A pathogen (from Greek pathos, suffering/emotion, an gene, to give birth to), infectious agent, or more commonly germ, is a biological agent that causes disease or illness to its host. However, pathogens can infect unicellular organisms from all of the biological kingdoms. The term pathogen is derived from the Greek "that which produces suffering." There are several substrates and pathways where by pathogens can invade a host; the principal pathways have different episodic time frames, but soil contamination has the longest or most persistent potential for harboring a pathogen.